Opaque, white film with low transparency made from a crystallizable thermoplastic

ABSTRACT

The present invention relates to an opaque, white film with a thickness of from 10 to 500 μm whose principal constituent is a crystallizable thermoplastic. It also comprises at least barium sulfate and at least one optical brightener. The barium sulfate and/or the optical brightener here is either incorporated directly into the thermoplastic during preparation of the polymer or fed as a masterbatch during film production. The novel film is particularly suitable for interior decoration, for constructing exhibition stands, for exhibition requisites, for displays, for placards, for labels, for protective glazing of machines or of vehicles, in the lighting sector, in the fitting out of shops or of stores, as a promotional requisite or a laminating material, or in applications associated with food or drink.

[0001] The invention relates to an opaque, white film with lowtransparency made from a crystallizable thermoplastic and having athickness of from 10 to 500 μm. The film comprises at least bariumsulfate and also an optical brightener, and has good orientability, lowtransparency, and also very good optical and mechanical properties. Theinvention further relates to a process for producing this film and tothe use of the film.

BACKGROUND OF THE INVENTION

[0002] Opaque films with a thickness of from 10 to 500 μm are wellknown.

[0003] The object of the present invention was to provide an opaque,white film with a thickness of from 10 to 500 μm which, besides havinggood orientability, good mechanical properties and good opticalproperties and a low Yellowness Index, above all has high whiteness andlow transparency.

[0004] The good optical properties include uniform, streak-freecoloration over the entire length and width of the film, low luminoustransmittance/transparency (≦30%), acceptable surface gloss (≧10), andalso a low Yellowness Index (dependent on thickness, ≦45 for 250 μmfilms and ≦20 for 50 μm films).

[0005] The good mechanical properties include a high modulus ofelasticity (E_(MD)≧3300 N/mm²; E_(TD)≧4200 N/mm²), and also good tearstrengths (in MD≧120 N/mm²; in TD≧170 N/mm²) and good longitudinal andtransverse elongations at break (in MD ≧120%; in TD≧50%).

[0006] Good orientability includes excellent capabilities of the filmfor orientation during its production, both longitudinally andtransversely, without break-offs.

[0007] The novel film should moreover be recyclable, that is to say thatany cut material arising during continuous film production can be fedback into the production operation as regrind, in particular withoutloss of optical or mechanical properties from the film, so that it canstill be used for interior applications and in constructing exhibitionstands, for example.

DESCRIPTION OF THE INVENTION

[0008] This object is achieved by an opaque white film with a thicknessof from 10 to 500 μm whose principal constituent is a crystallizablethermoplastic, wherein the film comprises at least barium sulfate and atleast one optical brightener, and the barium sulfate and/or the opticalbrightener either may be incorporated directly into the thermoplasticduring preparation of the polymer or is fed as a masterbatch during filmproduction.

[0009] The novel film comprises, as principal constituent, acrystallizable thermoplastic. Examples of suitable crystallizable orsemicrystalline thermoplastics are polyethylene terephthalate,polybutylene terephthalate and polyethylene naphthalate, and preferenceis given to polyethylene terephthalate.

[0010] For the purposes of the present invention, a crystallizablethermoplastic is

[0011] a crystallizable homopolymer;

[0012] a crystallizable copolymer;

[0013] a crystallizable compound;

[0014] a crystallizable recycled material, or

[0015] another type of crystallizable thermoplastic.

[0016] The novel film may have one layer, or else two or more layers,and it may have been coated with various copolyesters or with adhesionpromoters.

[0017] The film comprises at least barium sulfate as pigment, and theamount of pigment here is preferably from 0.2 to 40% by weight, based onthe weight of the crystallizable thermoplastic. The barium sulfate ispreferably fed to the thermoplastic by way of what is known asmasterbatch technology during film production.

[0018] The film comprises at least one optical brightener, and theamount of the optical brightener used here is from 10 to 50,000 ppm, inparticular from 20 to 30,000 ppm, particularly preferably from 50 to25,000 ppm, based on the weight of the crystallizable thermoplastic. Itis preferable for the optical brightener, too, to be fed to thethermoplastic by way of what is known as masterbatch technology duringfilm production.

[0019] The optical brighteners according to the invention are capable ofabsorbing UV radiation in the region from 360 to 380 nm and ofre-emitting this as longer-wavelength, visible blue-violet light.

[0020] Suitable optical brighteners are bisbenzoxazoles, phenylcoumarinsand bisstearylbiphenyls, in particular phenylcoumarin, and particularlypreferably triazine phenylcoumarin, obtainable as the product

Tinopal from Ciba-Geigy, Basle, Switzerland, or else

Hostalux KS (Clariant, Germany), or else

Eastobrite OB-1 (Eastman).

[0021] Besides the optical brightener, blue dyes soluble in polyestermay also be added if this is useful. Dyes which have proven successfulare cobalt blue, ultramarine blue and anthraquinone dyes, in particularSudan Blue 2 (BASF, Ludwigshafen, Germany).

[0022] The amounts of the blue dyes used are from 10 to 10,000 ppm, inparticular from 20 to 5000 ppm, particularly preferably from 50 to 1000ppm, based on the weight of the crystallizable thermoplastic.

[0023] It was highly surprising that the use of the abovementionedcombination of barium sulfate, optical brightener and, if present, bluedyes in the films in conjunction with the increased longitudinalstretching ratio during film production gave the desired result.

[0024] It is also very surprising that the cut film material can also bereused as regrind for production of the film without any adverse effecton the Yellowness Index of the film.

[0025] A preferred embodiment uses precipitated grades of bariumsulfate. Precipitated barium sulfate is obtained as a fine-particlecolorless powder from barium salts and sulfates or sulfuric acid, andthe particle size of the powder can be controlled via the conditions ofprecipitation. Precipitated barium sulfates may be prepared by the usualprocesses, which are described in Kunststoff-Journal 8, No. 10, 30-36and No. 11, 36-31 (1974).

[0026] The amount of barium sulfate is appropriately from 0.2 to 40% byweight, preferably from 0.5 to 30% by weight, particularly preferablyfrom 1 to 25% by weight, based on the weight of the thermoplastic.

[0027] The average particle size of the barium sulfate is relativelysmall and is preferably from 0.1 to 5 μm, particularly preferably from0.2 to 3 μm (Sedigraph method). The density of the barium sulfate usedis from 4 to 5 g/cm³.

[0028] In one particularly preferred embodiment, the novel filmcomprises, as principal constituent, a crystallizable polyethyleneterephthalate, and also from 1 to 25% by weight of precipitated bariumsulfate, appropriately with a particle diameter of from 0.4 to 1 μm,particularly preferably

Blanc fixe XR-HX or Blanc fixe HXH from Sachtleben Chemie.

[0029] The novel film also preferably comprises from 10 to 50,000 ppm ofan optical brightener soluble in the crystallizable thermoplastic,particularly preferably triazine phenylcoumarin (Tinopal, Ciba-Geigy,Basle, Switzerland), Hostalux KS, or else Eastobrite OB-1 (Eastman).

[0030] The surface gloss of the novel opaque white film, measured to DIN67530 (measurement angle 20°) is greater than or equal to 10, preferablygreater than or equal to 15.

[0031] The luminous transmittance (transparency) of the novel opaquewhite film, measured to ASTM-D 1003, is less than or equal to 30%,preferably less than or equal to 25%. The coloration is uniform andstreak-free over the entire running length and width of the film.

[0032] As a result of the synergistic action of the additives bariumsulfate, optical brightener and, if present, blue dye, in conjunctionwith optimized longitudinal stretching conditions, the film is whiter,that is to say has less yellow tinge, and has lower light transmittance,that is to say lower transparency, than a film provided only with bariumsulfate.

[0033] The longitudinal modulus of elasticity (ISO 527-1-2) of the novelopaque white film is greater than or equal to 3300 N/mm², preferablygreater than or equal to 3500 N/mm². Its transverse modulus ofelasticity (ISO 527-1-2) is greater than or equal to 4200 N/mm²,preferably greater than or equal to 4400 N/mm².

[0034] The standard viscosity SV (DCA) of the polyethyleneterephthalate, measured in dichloroacetic acid to DIN 53728, is from 600to 1100, preferably from 700 to 1000.

[0035] The intrinsic viscosity IV (DCA) is calculated from the standardviscosity SV (DCA) as follows:

IV (DCA)=6.67·10⁻⁴SV (DCA)+0.118

[0036] The opaque white polyethylene terephthalate film which comprisesat least barium sulfate, one optical brightener and, if desired, bluedyes may have either one layer or else two or more layers.

[0037] In the embodiment having two or more layers, the film has astructure of at least one core layer and at least one outer layer, andparticular preference is given here to a three-layer A-B-A or A-B-Cstructure.

[0038] A substantive factor for the embodiment having two or more layersis that the polyethylene terephthalate of the core layer has a standardviscosity similar to that of the polyethylene terephthalate of the outerlayer(s) which is/are adjacent to the core layer.

[0039] In one particular embodiment, the outer layers may also becomposed of a polyethylene naphthalate homopolymer or of a polyethyleneterephthalate-polyethylene naphthalate copolymer, or of a compound. Inthis particular embodiment, the thermoplastics of the outer layerslikewise have a standard viscosity similar to that of the polyethyleneterephthalate of the core layer.

[0040] In the embodiment having two or more layers, the barium sulfateand also the optical brightener and, if present, the blue dye arepreferably present in the core layer. However, modification of the outerlayers is also possible, if required.

[0041] In the embodiment having two or more layers, unlike in thesingle-layer embodiment, the amounts given for the additives are basedon the weight of the thermoplastics in the layer provided with theadditive(s).

[0042] There may also be provision of a scratch-resistant coating, acopolyester or an adhesion promoter on one or more sides of the film.

[0043] In addition, the novel film is easy to recycle without pollutionof the environment and without loss of mechanical properties, making itsuitable, for example, for use as short-lived promotional placards,labels or other promotional requisites.

[0044] An example of a production process for producing the novel filmis extrusion on an extrusion line.

[0045] According to the invention, the barium sulfate, the opticalbrightener and, if present, the blue dye may be incorporated directlywhen the thermoplastic polymer is prepared, or fed into the extruder byway of masterbatch technology during film production. It is preferablefor the barium sulfate, the optical brightener and, if present, the bluedye to be fed by way of masterbatch technology. The additives are fullydispersed in a solid carrier material. Carrier materials which may beused are the thermoplastic itself, e.g. the polyethylene terephthalate,or else other polymers sufficiently compatible with the thermoplastic.

[0046] It is important that the particle size and the bulk density ofthe masterbatch(es) are similar to the particle size and the bulkdensity of the thermoplastic, so that uniform dispersion is achieved,and, with this, uniform whiteness and thus uniform opacity.

[0047] The polyester films may be produced by known processes from apolyester with, if desired, other polymers, and also with the opticalbrightener, the barium sulfate, if desired with the blue dye, and/orwith a usual amount of from 0.1 to a maximum of 10% by weight of othercustomary additives, either in the form of monofilms or else in the formof, if desired, coextruded films having two or more layers and withidentical or differently constructed surfaces, where one surface mayhave provision of pigment, but no pigment is present in the othersurface. Known processes may also have been used to provide one or bothsurfaces of the film with a conventional functional coating.

[0048] In the preferred extrusion process for producing the polyesterfilm, the molten polyester material is extruded through a slot die andquenched on a chill roll, as a substantially amorphous prefilm. Thisfilm is then reheated and stretched longitudinally and transversely, ortransversely and longitudinally, or longitudinally, transversely andagain longitudinally and/or transversely. According to the invention,the stretching temperatures are from T_(g)+10 K to T_(g)+60 K (whereT_(g) is the glass transition temperature), the longitudinal stretchingratio according to the invention is from 2 to 5, in particular from 2.5to 4.5, and the transverse stretching ratio is from 2 to 5, inparticular from 3 to 4.5, and the ratio for any second longitudinalstretching carried out is from 1.1 to 3. The first longitudinalstretching may, if desired, be carried out simultaneously with thetransverse stretching (simultaneous stretching). This is followed by theheat-setting of the film at oven temperatures of from 200 to 260° C., inparticular from 220 to 250 C. The film is then cooled and wound up.

[0049] An entirely unexpected finding was that the process parametersfor the longitudinal stretching are a variable which significantlyaffects the optical properties (transparency) of the film. The processparameters for the longitudinal stretching include in particular thelongitudinal stretching ratio and the longitudinal stretchingtemperature. It was highly surprising that the transparency could bemarkedly affected by varying the longitudinal stretching ratio. If, forexample, a film plant produces a film whose transparency is above thevalues according to the invention, novel films with a lower transparencycan be produced by increasing the longitudinal stretching ratio duringthe longitudinal stretching procedure. A relative increase of 7% in thelongitudinal stretching ratio gave a relative reduction of from 15 to20% in transparency.

[0050] The surprising combination of excellent properties makes thenovel film highly suitable for a variety of different applications, suchas interior decoration, for constructing exhibition stands, forexhibition requisites, for displays, for placards, for labels, forprotective glazing of machines or of vehicles, in the lighting sector,in the fitting out of shops or of stores, as a promotional requisite ora laminating material, or in applications associated with food or drink.

[0051] The examples below illustrate the invention in more detail.

[0052] The following standards and methods are used here when testingindividual properties.

[0053] Test Methods

[0054] Surface Gloss

[0055] Surface gloss is measured with a measurement angle of 20° to DIN67530.

[0056] Luminous Transmittance/Transparency

[0057] For the purposes of the present invention, the luminoustransmittance/transparency is the ratio of total light transmitted tothe amount of incident light.

[0058] Luminous transmittance is measured using “Hazegard plus” testequipment to ASTM D 1003.

[0059] Surface Defects and Uniform Coloration

[0060] Surface defects and uniform coloration are determined visually.

[0061] Mechanical Properties

[0062] The modulus of elasticity, tear strength and elongation at breakare measured longitudinally and transversely to ISO 527-1-2.

[0063] SV (DCA) and IV (DCA)

[0064] The standard viscosity SV (DCA) is measured by a method based onDIN 53726 in dichloroacetic acid. The intrinsic viscosity (IV) iscalculated as follows from the standard viscosity (SV)

IV (DCA)=6.67·10⁻⁴SV (DCA)+0.118

[0065] Yellowness Index

[0066] The Yellowness Index YID is the deviation from the colorlesscondition in the “yellow” direction and is measured to DIN 6167.

[0067] Whiteness

[0068] Whiteness is determined by the method of Berger, generally usingmore than 20 mutually superposed layers of film. Whiteness is determinedwith the aid of the ®ELREPHO electrical reflectance photometer fromZeiss, Oberkochem, Germany, with standard illuminant C and 2° standardobserver. Whiteness is defined as W=RY+3RZ−3RX. W=whiteness, and RY, RZand RX=corresponding reflection factors using the Y, Z and Xcolor-measurement filter. The whiteness standard used is a bariumsulfate pressing (DIN 5033, Part 9). Hansl Loos, “Farbmessung” [Colormeasurement], Verlag Beruf und Schule, ltzehoe (1989) gives a detaileddescription.

[0069] In the examples and comparative examples below, each of the filmsis a single-layer opaque white film, produced on the extrusion linedescribed.

EXAMPLE 1

[0070] An opaque white film of 50 μm thickness was produced andcomprised polyethylene terephthalate (RT32, KoSa, Germany) as principalconstituent, 18% by weight of barium sulfate (Blanc fixe XR-HX,Sachtleben Chemie), 200 ppm of optical brightener (Tinopal, Ciba-Geigy,Basle) and 40 ppm of blue dye (Sudan Blue 2, BASF, Ludwigshafen).

[0071] The additives barium sulfate, optical brightener and blue dyewere added as masterbatches. The polyethylene terephthalate used forpreparing the masterbatches, had a standard viscosity SV (DCA) of from900 to 1100.

[0072] Masterbatch (1) was composed of clear polymer, 50% by weight ofbarium sulfate and 600 ppm of optical brightener. Masterbatch (2)comprised, in addition to clear polymer, 2000 ppm of blue dye.

[0073] Prior to extrusion, 36% by weight of masterbatch (1), 2% byweight of masterbatch (2) and 62% by weight of clear polymer were driedat a temperature of 150° C. and then melted in the extruder.

[0074] The longitudinal stretching ratio established during productionof the film was precisely 3.1.

EXAMPLE 2

[0075] Example 1 was repeated, except that the film was not providedwith blue dye.

EXAMPLE 3

[0076] The mixing specification corresponded to that of Example 2, butthe longitudinal stretching ratio was increased to 3.3, while thelongitudinal stretching temperatures remained unchanged.

EXAMPLE 4

[0077] A coextruded opaque white ABA film of 75 μm thickness wasproduced, A symbolizing the outer layers and B the core layer. Themixing specification of the 71 μm-thick core layer corresponded to themixing specification of Example 2. The outer layers, of 2 μm thickness,comprised 93% by weight of clear polymer and also 7% by weight of amasterbatch comprising, in addition to clear polymer, 10,000 ppm ofsilicon dioxide (®Sylobloc, Grace, Germany). This film had particularlyhigh surface gloss. The longitudinal stretching ratio was 3.3.

COMPARATIVE EXAMPLE 1

[0078] Example 3 was repeated, except that the longitudinal stretchingratio was reduced to 2.8 but the longitudinal stretching temperaturesestablished remained unchanged.

COMPARATIVE EXAMPLE 2

[0079] Comparative Example 1 was repeated, except that the film was notprovided with optical brightener. The film comprised only 18% by weightof barium sulfate which was incorporated directly during preparation ofthe polymer. The standard viscosity of the polymer comprising bariumsulfate was 810.

[0080] The opaque white PET films produced in Examples 1 to 4 and thefilms of Comparative Examples 1 and 2 had the property profilesillustrated in the table below: TABLE Comp. Comp. Properties Ex. 1 Ex. 2Ex. 3 Ex. 4 Ex. 1 Ex. 2 Thickness [μm] 50 50 50 75 50 50 Surface gloss20 20 20 165 20 20 side 1 (Measurement angle 20°) 20 20 20 165 20 20side 2 Luminous trans- 20 20 16 12 25 24 mittance/transparency [%]Yellowness Index (YID) 12 14 14 18 15 24 Whiteness by Berger method 9391 91 90 91 84 [%] Longitudinal modulus of 3600 3600 3650 3650 3350 3500elasticity [N/mm²] Transverse modulus of 5200 5300 5200 5300 5300 5300elasticity [N/mm²] Longitudinal tear strength 150 155 155 150 150 150[N/mm²] Transverse tear strength 240 240 235 240 250 250 [N/mm²]Longitudinal elongation at 180 175 175 170 180 175 break [%] Transverseelongation at 70 75 75 75 70 80 break [%] Coloration brill-iantbrilliant brill-iant brill-iant brill-iant tinged white white whitewhite white yellow

What is claimed is:
 1. An opaque white film with a thickness of from 10to 500 μm whose principal constituent is a crystallizable thermoplastic,wherein the film comprises at least barium sulfate and at least oneoptical brightener, and the barium sulfate or the optical brightener orthe barium sulfate and the optical brightener is either incorporateddirectly into the thermoplastic during preparation of the polymer or fedas a masterbatch during film production.
 2. The opaque white film asclaimed in claim 1, wherein the crystallizable thermoplastic has beenselected from the group consisting of polyethylene terephthalate,polybutylene terephthalate and polyethylene naphthalate.
 3. The opaquewhite film as claimed in claim 1, wherein from 0.2 to 40% by weight ofbarium sulfate, based on the weight of the crystallizable thermoplastic,is present in the film and the barium sulfate is fed to thethermoplastic by way of masterbatch technology during film production.4. The opaque white film as claimed in claim 1, wherein from 10 to50,000 ppm, of optical brightener, based on the weight of thecrystallizable thermoplastic, is present in the film and the opticalbrightener is fed to the thermoplastic by way of masterbatch technologyduring film production.
 5. The opaque white film as claimed in claim 4,wherein the optical brightener has been selected from the groupconsisting of bisbenzoxazoles, phenylcoumarins and bisstearylbiphenyls.6. The opaque white film as claimed in claim 1, wherein apolyester-soluble blue dye selected from the group consisting of cobaltblue, ultramarine blue and anthraquinone dyes, is also present in thefilm, and the amount of blue dye is from 10 to 10,000 ppm, based on theweight of the crystallizable thermoplastic.
 7. The opaque white film asclaimed in claim 1, wherein the barium sulphate is present asprecipitated barium sulfate in the film and is in the form offine-particle colorless powder with an average grain size of from 0.1 to5 μm, measured by the Sedigraph method.
 8. The opaque white film asclaimed in claim 1, wherein the surface gloss of the film measured toDIN 67530 (measurement angle 20°) is greater than or equal to 10, andthe luminous transmittance (transparency) of the film, measured toASTM-D 1003 is less than or equal to 30%.
 9. The opaque white film asclaimed in claim 1, wherein the film has one or more layers, and theembodiment having more than one layer comprises at least one core layerand at least one outer layer.
 10. The opaque white film as claimed inclaim 9, wherein, in the embodiment having more than one layer, thebarium sulfate and the optical brightener are present in the core layer.11. The opaque white film as claimed in claim 10, wherein the outerlayer(s), too, have been provided with barium sulfate and opticalbrightener.
 12. The opaque white film as claimed in claim 1, wherein ascratch-resistant coating, a copolyester or an adhesion promoter hasbeen provided on at least one side of the film.
 13. A process forproducing the opaque white film as claimed in claim 1 by extrusion, inwhich the thermoplastic material barium sulfate and optical brightenermelted in an extruder is compacted, extruded through a slot die andquenched on a chill roll, as a substantially amorphous prefilm, and thenreheated and stretched longitudinally and transversely, or transverselyand longitudinally, or longitudinally, transversely and againlongitudinally and/or transversely, which comprises establishing thestretching temperatures at from T_(g)+10 K to T_(g)+60 K andestablishing a longitudinal stretching ratio of from 2 to 5, and atransverse stretching ratio of from 2 to 5, and then heat-set the film.14. The process as claimed in claim 13, wherein the heat-setting of thefilm is carried out at oven temperatures of from 200 to 260° C.